1. Field of the Invention
This invention relates to polymeric compositions useful in coating applications. More specifically, this invention relates to polymer compositions having good melt flow under zero shear conditions and useful in coating various substrates such as glass bottles which are employed as containers particularly in the carbonated beverage and beer industries.
2. Description of the Prior Art
The hazards of using glass containers, particularly glass bottles which contain beer or carbonated beverages, are well known. Breakage of such bottles often takes place due to the internal pressure exerted by the pressurized gas in the carbonated beverage or beer as well as by dropping the bottles and other impacts caused by external forces which occur not only in the course of production and distribution of the bottled product, but also as a result of handling of the bottled product by consumers. Such breakage may result in injuries to the human body.
Coatings have been applied to such bottles in order to prevent scattering of the broken glass upon breakage. In this connection, see West German Patent No. DT2,636,157 and U.S. Pat. No. 3,823,032. The latter patent discloses that thermoplastic elastomers such as block copolymers of styrene and butadiene are useful in coating glass bottles. Thermoplastic elastomeric compositions, having improved environmental resistance, are disclosed in U.S. Pat. No. 2,686,365. These polymeric compositions comprise
(a) about 80 to 99 percent of block copolymers having at least two mono alpha alkenyl arene polymer blocks and at least one conjugated diene polymer block and
(b) about 20 to 1 percent of a selectively hydrogenated block copolymer having, prior to hydrogenation, at least one mono alpha alkenyl arene polymer block and at least one conjugated diene polymer block, said block copolymer having been selectively hydrogenated to saturate at least 80 percent of the olefinic double bonds and 0 to 25 percent of the aromatic double bonds.
Because of the rather recent concern with environmental considerations, it is especially desirable to be able to apply protective coatings to various substrates such as glass bottles in a non-polluting manner. The use of solid powder particles to coat these substrates eliminates the need for solvent and the accompanying environmental problems. In this connection, see U.S. Pat. No. 3,737,401.
When the powder particles are deposited upon a substrate such as a glass bottle, the particles must be heated to provide a uniform molten coating on the glass bottle. The coating is then cooled to provide a uniform solid coating on the bottle.
When certain thermoplastic elastomers such as styrene/butadiene/styrene block copolymers are used as a major component of the powder particles, the melt flow of the thermoplastic elastomers is not sufficient to provide a uniform molten coating. Although it is known to use copolymers of alphamethylstyrene and styrene with certain elastomers to produce pressure sensitive adhesives and to use such additives in hot melt coatings (see U.S. Pat. No. 3,932,332), the use of this additive does not provide sufficient flow in thermoplastic elastomers at temperatures below that at which the thermoplastic elastomer discolors.
It is also known to use hydrogenated aromatic hydrocarbon petroleum resins as tackifiers for polymers such as styrene/isoprene/styrene block copolymers in a hot melt adhesive. For example, U.S. Defensive Publication No. T917008 discloses a hot melt adhesive containing a tackifier which is derived from a polymerized cracked naphtha fraction and having a boiling point between -10.degree. and 280.degree. C. This fraction contains polymerizable unsaturated hydrocarbons, inert paraffins and alkyl benzenes. The polymerizable unsaturated hydrocarbons present in the fraction include aromatic olefins, cyclic olefins, cyclic diolefins, aliphatic olefins and aliphatic diolefins. Of the unsaturated hydrocarbon content, at least about 50% by weight, consists of a mixture of aromatic olefins, cyclic olefins and cyclic diolefins. The aromatic olefins present in the petroleum fraction include styrene, alpha-methylstyrene, vinyl toluene, vinyl xylene, propenyl benzene, indene, methyl indene, ethyl indene, and the like. The cyclic olefins and cyclic diolefins include cyclopentene, cyclopentadiene, dicyclopentadiene, cyclohexene, cyclooctene, and the like. The aliphatic olefins and aliphatic diolefins include butene, butadiene, pentene, pentadiene, octadiene, and the like.
The problem of melt flow of thermoplastic elastomers exists not only with respect to coatings useful for glass bottles but also for other coating applications and for large part molding such as roto casting.
A particularly efficacious method for solving the above-identified problems is disclosed in U.S. patent application Ser. No. 844,963, now U.S. Pat. No. 4,163,031, l entitled "Powder Composition and Method of Preparation" and filed concurrently herewith by Martin J. Hannon and Alex S. Forschirm. However, the powder particles produced by this method tend to coalesce and become somewhat tacky, lumpy materials which are somewhat difficult to spray. These powders tend to produce coatings which have the appearance of an orange peel. Since these coatings tend to be rather lumpy, their appearance, adhesive properties and glass fragment retention are not as good as they would be if such lumpiness could be eliminated.
This tendency of thermoplastic elastomers to stick together, coalesce, agglomerate and/or exhibit "blocking," results from an undesired adhesion between touching particles such as occurs under moderate pressure during storage of the particles. Particularly, storage under somewhat elevated temperatures, and under pressures caused by stacking bales or packages of polymer, create conditions favorable for such agglomeration. If the particles of the polymers agglomerate, then it frequently becomes necessary to grind, crush, or otherwise masticate the mass in order to reseparate the particles or to again produce a utilizable particulate material. Such mechanical treatment is burdensome and undesirable because of inconvenience, added labor and time, cost, possible contamination of the elastomer, and possibly in some instances even degradation of some of the polymeric products due to the additional working, temperature, and the like.
In attempting to ease this problem, it is known to apply a dusting agent such as carbon black, talc, zinc stearate, rice flour, chalk, magnesium oxide, infusorial earth, or the like, to the particles in an effort to counteract the natural tackiness or blockiness of the particulate-form polymers. All of these dusting agents, however, have some objectionable characteristics. For example, adding color to the natural polymer may be undesirable for some purposes. The dusting agents may be objectionable for some end uses, such as in clear coatings where the presence of such agents could cause haziness. Silica powder and some grades of talc may possibly pose health hazards under some circumstances that will restrict their use. Stearate powders at levels sufficient to combat tackiness may adversely affect polymer performance properties such as tack, adhesion, optical clarity, and the like.
For a more detailed discussion of the disadvantages of prior art attempts to solve this general problem, see U.S. Pat. Nos. 3,528,841 and 4,027,067 and British Specification No. 1,200,532. The disclosure of U.S. Pat. No. 4,027,067 is hereby incorporated by reference.
The search has continued for improved powder coating compositions useful in coating applications such as in coating glass bottles to protect against the scattering of broken glass. This invention was made as a result of that search.